Half bearing

ABSTRACT

A half bearing has a backing member and at least one metallic overlay, which is applied by electron beam vapor deposition and which comprises at least one finely dispersed component in a matrix material, the atomic weight of which component is greater than that of the matrix material, wherein the concentration of the finely dispersed component (7) decreases continuously from the apex area (8) of the half bearing (1) towards the area (9) of the partial surfaces. The method is characterized in that, during the coating process, an inert gas pressure of 0.1 to 5 Pa is set in the apex area of the bearing shell.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to a half bearing, comprising a backing member andat least one metallic overlay, which is applied by means of electronbeam vapour deposition and which comprises at least one finely dispersedcomponent in a matrix material, the atomic weight of which component isgreater than that of the matrix material.

2. Related Prior Art

In general, sliding elements used for such purposes comprise multilayercomposite systems of the following construction: steel support memberserving as backing material, bearing metal layer of a Cu, Al or whitemetal alloy and a so-called sliding or third layer or overlay, which maybe applied either by an electroplating process (E. Romer:Three-component bearings of GLYCO 40; GLYCO Engineering Report August1967) or by a cathodic sputtering process as described in EP 0 256 226B1. Layers applied by electroplating, which are generally based on Pb orSn, exhibit the disadvantages of frequently inadequate corrosionresistance and low wear resistance. Furthermore, the electroplatingprocess is in itself dubious from the environmental point of the view.

Where overlays are applied by the sputtering method, a considerable costfactor is introduced with respect to the complete sliding element, owingto the low deposition rates achievable therewith and the high technicalcomplexity of the equipment needed.

GB 2270 927 describes aluminium alloys in which the SN content of theentire layer may be constant and lie between 10 and 80%. From Table 1 onpages 10 and 11 of this application it may be seen that, as the tincontent increases, the possible limit load before the bearingdemonstrates a tendency to corrode increases, while, on the other hand,the load-carrying capacity drops again drastically from a certain tincontent. This specification does not contain any suggestions forimproving running-in behaviour. Sputtering is mentioned in thisapplication as a production process for applying the overlay.

EP 0 376 368 B1 describes a very complex process for producing a bearingwhich is distinguished by good emergency running and running-inproperties. This application also relates to aluminium-tin alloys, whichare applied by means of a sputtering process. The nub of thisapplication is that the particles incorporated in the metallic basematerial of the bearing alloy obey the rules of standard randomdistribution with respect to the diameter thereof, and that up to 1.0wt. % oxygen is incorporated in the overlay, the micro-hardness of theoverlay diminishing after heat treatment. In this way, theembeddability, emergency running properties and insensitivity tocorrosion are improved.

WO 91/00375 describes a bearing whose overlay consists of a basematerial (e.g. aluminium) with a second phase (e.g. tin) dispersedfinely therein. Here too a sputtering process is used. The aim of thisinvention is to produce a bearing whose overlay structure is such thatthe content of the second phase (e.g. tin) in the overlay increases inaccordance with the thickness of the overlay continuously from 0% in thebottom layers to 100% in the top layers. This is effected on the onehand by the use of several targets of differing compositions or ofvarying sputtering parameters during coating. Overlays produced in thisway exhibit very good properties from the point of view of their wearand fatigue behaviour, which is achieved, however, by using a verycomplex process.

It is known from DE 195 14 835 A1 and 195 14 836 A1 to deposit overlayson concavely curved sliding elements by means of electron beam vapourdeposition, wherein in both specifications the formation of particularlayer thickness profiles is a priority. In order to achieve a uniformlayer thickness for half bearings, according to DE 195 14 835 A1 theevaporating apparatus and backing member are moved relative to oneanother in linear manner and at different speeds during evaporationcoating. To this end, appropriate adjusting means are required insidethe coating chamber. The intention of DE 195 14 836 A1, on the otherhand, is to produce a non-uniform layer thickness. The layer thicknessof the sliding element is at its greatest in the apex area and reducescontinuously towards the partial surfaces. In order to achieve this, themethod provides that a distance be set between the evaporating apparatusand the apex area of the half bearing of 150 to 350 mm, that duringevaporation coating of the layer the evaporating apparatus the backingmember be positioned in fixed relation to each other and that thecondensation rate for deposition in the apex area be set at at least 80nm/s.

A method is known from DE 36 06 529 A1 for producing multilayermaterials or multilayer workpieces by the vapour deposition of at leastone metallic material onto a metallic substrate, an electron beam vapourdeposition process likewise being used to apply the overlay. The methodis carried out in a residual gas atmosphere under pressures ranging from10⁻² -10⁻³ mbar, wherein the material is dispersion-hardened ordispersion-strengthened simultaneously with the vapour deposition.Coating rates are set at approximately 0.3 μm/s. During vapourdeposition, the substrate is kept at a temperature between 200° C. and800° C. The temperature of the substrate is 200° C. to 300° C. forvapour deposition of aluminium alloys and in the range of from 500° C.to 700° C. for vapour deposition of copper-lead alloys. Theload-carrying capacity of the layers produced according to this methodis markedly better than that of layers produced by powder-metallurgicalmethods. A priority of this application is to produce a defined hardphase content in the overlay by dispersion strengthening, e.g. byproducing oxides during vapour deposition.

These three specifications relating to electron beam vapour depositiondo not give any indications as to how to achieve different distributionsof the alloy components. The load-carrying capacity or running-inbehaviour are not adequate for some applications.

SUMMARY OF THE INVENTION

The object of the invention is to provide a half bearing which isdistinguished, preferably in its highly loaded areas, by good emergencyrunning and running-in behaviour combined with high limit loads beforethe onset of bearing corrosion. It is also the object of the inventionto provide an economic method based on electron beam vapour depositionfor the production of such half bearings, which method additionallyensures in a simple manner a uniform layer thickness over the entirecircumference of the bearing shell.

The half bearing is characterised in that the concentration of thefinely dispersed component decreases continuously from the apex area ofthe half bearing towards the area of the partial surfaces.

An overlay constructed in this way has the advantage that, in the mosthighly loaded area, namely the apex area, that alloy component which hasa decisive positive influence on emergency running and running-inbehaviour is present in the highest concentration. Another advantageconsists in the fact that the relatively expensive, finely dispersedcomponent is present in a correspondingly high concentration only inthose areas where it is chiefly required in relation to emergencyrunning and running-in behaviour.

Since the sliding properties of the most highly loaded areas affect theservice life of the entire plain bearing, an increase in service life isalso ensured by an increase in the concentration of the finely dispersedalloy component in the apex area.

The concentration of the finely dispersed component in the apex area ispreferably higher in the apex area than in the area of the partialsurfaces by a factor of 1.2 to 1.8, preferably 1.3 to 1.6.

According to a first embodiment, the concentration of the finelydispersed component is constant over the thickness of the overlay.

This concentration distribution in the circumferential direction mayalso be combined, according to a second embodiment, with a differentconcentration over the layer thickness, wherein the concentration of thefinely dispersed component preferably increases continuously from thelower side, i.e. the side in the vicinity of the backing member, to theupper area of the overlay. This embodiment of the overlay is selectedwhen the half bearing counter-member exhibits high levels of surfaceroughness, as is the case with cast shafts, for instance.

The concentration of the finely dispersed component in the upper area ofthe overlay is preferably up to 2 times greater than in the lower area.

The concentration of the finely dispersed component in the apex areaamounts advantageously to between 10 and 70 wt. %.

The matrix material preferably consists of aluminium, wherein the finelydispersed component may consist of tin, lead, bismuth and/or antimony.As its other alloy component, the overlay may comprise copper, zinc,silicon, manganese and/or nickel individually or in combination in aproportion of up to 5 wt. %.

The backing members may comprise steel supporting shells, or may consistof steel/CuPbSn composite materials or steel/aluminium or steel/whitemetal composite materials. Preferred alloy systems constituting theoverlay are AlSnCu, AlSnPb and AlSnSi. In the case of an overlayconsisting of a tin alloy, the tin content in the overlay decreases fromthe apex towards the partial surfaces of the sliding element, i.e. theoverlay comprises areas with high and low tin contents. This makes itpossible, for the first time, to exploit the advantages of high and lowtin contents in the overlay simultaneously. While the area with high tincontent ensures good running-in behaviour of the sliding element, theareas with low tin content ensure that the sliding element exhibits ahigh load-carrying capacity.

The thickness of the overlay is preferably uniform over the entirecircumference.

The method of producing such half bearing overlays provides that a gaspressure of 0.1 to 5 Pa be set during the coating process in the apexarea of the bearing shell.

The gas molecules between the vaporiser crucible and the surface to becoated effect scattering of the alloy components which varies during thevapour deposition process.

The scattering angle or degree of scattering depends, for kineticreasons, on the relative density of the individual evaporated alloyelements. The consequence thereof is that heavy elements, such as tinfor example, are less strongly scattered than lighter elements, forexample aluminium. The result of these scattering processes is that theheavy elements are deposited in the apex area of the half bearing inhigher concentration than in the area of the partial surfaces. Thisscattering by means of gas molecules makes it possible to vary thecomposition of the overlay within broad limits depending on the pressurerange in which electron beam vapour deposition is effected.

Contrary to the opinion held in specialist circles, the layers producedwith gas scattering are surprisingly compact and superior, with regardto their wear resistance properties and load-carrying capacity, tosliding elements produced conventionally or by electron beam vapourdeposition without supplementary measures.

It has moreover surprisingly emerged that, in addition to the obtainmentof different concentrations, it is also simultaneously possible toobtain a uniform layer thickness, such that supplementary measures, suchas are known from DE 195 14 835 A1, may be dispensed with.

The production process is thereby simplified considerably.

The gas pressure may preferably be kept constant at 0.05 Pa during thecoating process.

The method may be further modified by varying the gas pressurecontinuously during the coating process. If the gas pressure is variedas a function of vapour deposition time, a variation in composition overthe layer thickness is also achieved, in addition to the graduatedstructure of the overlay in the circumferential direction.

The gas pressure is preferably increased continuously from 0.1 Pa at thebeginning to 1 Pa at the end of the vapour deposition process. Theincrease in gas pressure has the effect that the alloy component havingthe low atomic weight is scattered increasingly to a greater degree thanthe heavy alloy elements, so increasing the difference in concentrationbetween the apex area and the area of the partial surfaces during thecourse of the process. In this way, the concentration of the alloycomponents also varies over the layer thickness.

The inert gas used is preferably argon, helium or neon. Theperpendicular distance between the half bearing and the vaporisercrucible must be set at 2 to 7 times the half bearing diameter and thecoating rate in the apex area must be set at at least 20 nm/s.

THE DRAWINGS

The invention is described in more detail below with the aid of anexemplary embodiment and the drawings, in which;

FIG. 1 is a perspective representation of a half bearing,

FIG. 2 is a diagram showing the alloy composition of the overlay in theapex area,

FIG. 3 is a diagram showing the alloy composition of the overlay in thearea of the partial surfaces,

FIG. 4 is a diagram showing the limit loads, achievable in an Underwoodtest, for half bearings with overlays produced according to theinvention, in comparison with conventional ternary and two-componentbearings.

DETAILED DESCRIPTION

FIG. 1 shows a half bearing 1 having a backing member 2 and an overlay6. The backing member 2 consists of a steel supporting shell 3, ontowhich a CuPbSn alloy 4 has been applied by a casting or sinteringprocess together with a diffusion barrier layer 5. The carbon content ofthe steel is between 0.03% and 0.3%.

After various annealing and forming processes known per se, halfbearings are produced from a strip by pressing strip pieces of aspecific length. After surface machining of these bearings by drillingor rolling, the bearing shells are provided with a diffusion barrierlayer 5 of nickel or a nickel alloy by an electroplating or a PVDprocess. The backing member is then degreased and introduced into avacuum evaporation installation. Further surface cleaning or activationis effected therein by a sputtering and etching process.

After evacuation of the coating chamber, the latter is flooded withargon, the pressure being set at approximately 1 Pa. The backing member2 is then coated by electron beam vapour deposition of AlSn20Cu from avaporiser crucible by means of an axial electron gun. The thickness ofthe deposited overlay 6 of AlSn20Cu is approximately (16±4)

During the vapour deposition process, the argon pressure was keptconstant at 1 Pa, the temperature of the backing member was 190° C. to200° C. and the power of the electron gun amounted to from 40 to 60 kW.The deposition rate was at least 20 nm/s.

In the apex area 8, the overlay 6 exhibits a markedly higherconcentration of tin than in the area 9 of the partial surfaces. The tinparticles are designated by the dots 7. The higher concentration isillustrated by a higher dot density in the apex area 8.

FIGS. 2 and 3 show the alloy composition in the apex area (FIG. 2) andin the area of the partial surfaces (FIG. 3). The tin content wasdetermined by scanning electron microscope using EDX for a particulararea of the vapour-deposited AlSn20Cu layer. The concentration of tin inthe apex area 8 is higher than in the area of the partial surfaces by afactor of 1.4, the values being determined from integration of the tinpeaks.

FIG. 4 directly compares the limit loads, obtainable using the Underwoodtest bench, of the bearing shells produced with graduated overlaysaccording to the invention with conventional ternary and two-componentbearings. An aluminium two-component bearing (bar A) with an overlay ofAlSn20Cu (taken as 100%) was selected as the basis for these tests.Higher load-carrying capacities are provided by an AlSn-basedtwo-component bearing (bar B), whose matrix is strengthened by the alloyelements nickel and manganese. The ternary bearing (bar C) with asteel/lead-bronze/electrodeposited layer (PbSn10Cn5) structure acceptsloads which lie between the above-described two-component bearings. AsFIG. 4 shows, the plain bearings (bar D) evaporation coated according tothe invention are superior. to conventional bearing systems with regardto their load-carrying capacity.

What is claimed is:
 1. A half bearing comprising a backing member and at least one metallic overlay, which is applied by electron beam vapour deposition and which comprises at least one finely dispersed component in a matrix material, the atomic weight of which component is greater than that of the matrix material characterised in thatthe concentration of the finely dispersed component (7) decreases continuously from the apex area (8) of the half bearing (1) towards the area (9) of the partial surfaces.
 2. A half bearing according to claim 1, characterised in that the concentration of the finely dispersed component (7) in the apex area (8) is higher than in the area (9) of the partial surfaces by a factor of 1.2 to 1.8.
 3. A half bearing according to claim 1, characterized in that the concentration of the finely dispersed component (7) is constant over the thickness of the overlay (6).
 4. A half bearing according to claim 1, characterized in that the concentration of the finely dispersed component (7) increases continuously from the lower area to the upper area of the overly (6).
 5. A half bearing according to claim 4, characterized in that the concentration of the finely dispersed component (7) is up to twice as great in the upper area of the overlay (6) as in the lower area.
 6. A half bearing according to claim 1, characterized in that the concentration of the finely dispersed component (7) in the apex area (8) is between 10 and 70 wt. %.
 7. A half bearing according to claim 1, characterized in that the matrix material comprises aluminum and in that the finely dispersed component (7) consists of tin, lead, bismuth and/or antimony.
 8. A half bearing according to claim 1, characterized in that, as its other alloy component, the overlay (6) comprises copper, zinc, silicon, manganese and/or nickel individually or in combination in a proportion of up to 5 wt. %.
 9. A half bearing according to claim 1, characterized in that the thickness of the overlay (6) is uniform over the entire circumference. 